Hi Linh,
This is an interesting observation.
The FHL tendon sheath communicates directly with the ankle joint much the same as the long head of biceps tendon communicates with the glenohumeral joint. This means that a small amount of fluid is really commonly seen in the proximal tendon sheath near the MTJ and also in the know of henry area of FHL in normal volunteers.
The tibias posterior tendon sheath also nearly always contains a small amount of fluid in normal volunteers near the insertion and commonly in the proximal tendon sheath although tib post does not communicate with the ankle joint.
So in these 2 tendons I don’t really associate a small amount of fluid with pathology, but rather as a normal finding.
BUT if the patient has clinical symptoms over the tendon and the sheath or if I see any wasculatity of either the tendon or the sheath I would consider this to be tenosynovitis.
FDL is a little different and I don’t really see fluid in this sheath very often.
For me to diagnose an MTJ tear I would need to see a muscle abnormality at the MTJ itself and remember there is an intra-muscular extension of these tendons and this is where these injuries occur. Fluid in the proximal sheath in my mind does not indicate that the pathology is in that part of the sheath as the fluid will move with posture and gravity / transducer pressure. Fluid in the sheath is just that and can be a normal finding as I described or associated with a tenosynovitis. I don’t really associate a sheath effusion with an MTJ tear.
In your case the tendon swelling and mild hyperaemia indicates tenosynovitis as the most likely diagnosis in my mind.
Steve